code: def add(self, chart, tab_name):
      chart.tab_name = tab_name
      self._charts.append(chart)
      return self
  
file_path: pyecharts/test/test_mixins.py
imports: from unittest.mock import patch
  from nose.tools import assert_equal
  from pyecharts import options
  from pyecharts.charts import Bar, Line
  from pyecharts.charts.mixins import CompositeMixin
label_desc: Use image node_id 2 for calling the CustomCompositeChart obj's underlying member method code with example usage: obj.add(chart, tab_name) and returns: self